In the present project the influence of cyclic nucleotides (cAMP and cGMP) on the cell diffusion of Lucifer Yellow CH in mammalian cardiac fibers will be investigated. For this, the cut-end method will be used. Our main objective is to elucidate the possible role of cyclic nucleotides on the physiological regulation of junctional permeability in cardiac fibers. As many hormones, such as epinephrine and glucagon enhance the intracellular concentration of cAMP in heart cells it is possible to visualize that part of the hormonal action is related to an increase in the exchange of ions and molecules between heart cells. It is extremely important to know how the junctional permeability is regulated under physiological conditions because impairment of electrical synchronization and conduction in heart muscle can be visualized as a natural consequence of a change in junctional conductance.